DESCRIPTION

The Internet Systems Consortium DHCP Client, dhclient, provides a
means for configuring one or more network interfaces using the Dynamic
Host Configuration Protocol, BOOTP protocol, or if these protocols
fail, by statically assigning an address.

OPERATION

The DHCP protocol allows a host to contact a central server which
maintains a list of IP addresses which may be assigned on one or more
subnets. A DHCP client may request an address from this pool, and
then use it on a temporary basis for communication on network. The
DHCP protocol also provides a mechanism whereby a client can learn
important details about the network to which it is attached, such as
the location of a default router, the location of a name server, and
so on.

On startup, dhclient reads the
dhclient.conf
for configuration instructions. It then gets a list of all the
network interfaces that are configured in the current system. For
each interface, it attempts to configure the interface using the DHCP
protocol.

In order to keep track of leases across system reboots and server
restarts, dhclient keeps a list of leases it has been assigned in the
dhclient.leases(5) file. On startup, after reading the dhclient.conf
file, dhclient reads the dhclient.leases file to refresh its memory
about what leases it has been assigned.

When a new lease is acquired, it is appended to the end of the
dhclient.leases file. In order to prevent the file from becoming
arbitrarily large, from time to time dhclient creates a new
dhclient.leases file from its in-core lease database. The old version
of the dhclient.leases file is retained under the name
dhclient.leases~
until the next time dhclient rewrites the database.

Old leases are kept around in case the DHCP server is unavailable when
dhclient is first invoked (generally during the initial system boot
process). In that event, old leases from the dhclient.leases file
which have not yet expired are tested, and if they are determined to
be valid, they are used until either they expire or the DHCP server
becomes available.

A mobile host which may sometimes need to access a network on which no
DHCP server exists may be preloaded with a lease for a fixed
address on that network. When all attempts to contact a DHCP server
have failed, dhclient will try to validate the static lease, and if it
succeeds, will use that lease until it is restarted.

A mobile host may also travel to some networks on which DHCP is not
available but BOOTP is. In that case, it may be advantageous to
arrange with the network administrator for an entry on the BOOTP
database, so that the host can boot quickly on that network rather
than cycling through the list of old leases.

COMMAND LINE

The names of the network interfaces that dhclient should attempt to
configure may be specified on the command line. If no interface names
are specified on the command line dhclient will normally identify all
network interfaces, eliminating non-broadcast interfaces if
possible, and attempt to configure each interface.

It is also possible to specify interfaces by name in the
dhclient.conf(5)
file. If interfaces are specified in this way, then the client will
only configure interfaces that are either specified in the
configuration file or on the command line, and will ignore all other
interfaces.

If the DHCP client should listen and transmit on a port other than the
standard (port 68), the
-p
flag may used. It should be followed by the udp port number that
dhclient should use. This is mostly useful for debugging purposes.
If a different port is specified for the client to listen on and
transmit on, the client will also use a different destination port -
one less than the specified port.

The DHCP client normally transmits any protocol messages it sends
before acquiring an IP address to, 255.255.255.255, the IP limited
broadcast address. For debugging purposes, it may be useful to have
the server transmit these messages to some other address. This can
be specified with the
-s
flag, followed by the IP address or domain name of the destination.

For testing purposes, the giaddr field of all packets that the client
sends can be set using the
-g
flag, followed by the IP address to send. This is only useful for testing,
and should not be expected to work in any consistent or useful way.

The DHCP client will normally run in the foreground until it has
configured an interface, and then will revert to running in the
background. To run force dhclient to always run as a foreground
process, the
-d
flag should be specified. This is useful when running the client
under a debugger, or when running it out of inittab on System V
systems.

The dhclient daemon creates its own environment when executing the
dhclient-script to do the grunt work of interface configuration.
To define extra environment variables and their values, use the
-e
flag, followed by the environment variable name and value assignment,
just as one would assign a variable in a shell. Eg:
-eIF_METRIC=1

The client normally prints a startup message and displays the
protocol sequence to the standard error descriptor until it has
acquired an address, and then only logs messages using the
syslog (3)
facility. The
-q
flag prevents any messages other than errors from being printed to the
standard error descriptor.

The client normally doesn't release the current lease as it is not
required by the DHCP protocol. Some cable ISPs require their clients
to notify the server if they wish to release an assigned IP address.
The
-r
flag explicitly releases the current lease, and once the lease has been
released, the client exits.

If the client is killed by a signal (for example at shutdown or reboot)
it won't execute the
dhclient-script (8)
at exit. However if you shut the client down gracefully with
-r
or
-x
it will execute
dhclient-script (8)
at shutdown with the specific reason for calling the script set.

The
-1
flag will cause dhclient to try once to get a lease. If it fails, dhclient
exits with exit code two.

The DHCP client normally gets its configuration information from
/etc/dhcp3/dhclient.conf,
its lease database from
/var/lib/dhcp3/dhclient.leases,
stores its process ID in a file called
/var/run/dhclient.pid,
and configures the network interface using
/sbin/dhclient-script
To specify different names and/or locations for these files, use the
-cf,-lf,-pf
and
-sf
flags, respectively, followed by the name of the file. This can be
particularly useful if, for example,
/var/lib/dhcp3
or
/var/run
has not yet been mounted when the DHCP client is started.

The DHCP client normally exits if it isn't able to identify any
network interfaces to configure. On laptop computers and other
computers with hot-swappable I/O buses, it is possible that a
broadcast interface may be added after system startup. The
-w
flag can be used to cause the client not to exit when it doesn't find
any such interfaces. The
omshell (1)
program can then be used to notify the client when a network interface
has been added or removed, so that the client can attempt to configure an IP
address on that interface.

The DHCP client can be directed not to attempt to configure any interfaces
using the
-n
flag. This is most likely to be useful in combination with the
-w
flag.

The client can also be instructed to become a daemon immediately, rather
than waiting until it has acquired an IP address. This can be done by
supplying the
-nw
flag.

CONFIGURATION

OMAPI

The DHCP client provides some ability to control it while it is
running, without stopping it. This capability is provided using OMAPI,
an API for manipulating remote objects. OMAPI clients connect to the
client using TCP/IP, authenticate, and can then examine the client's
current status and make changes to it.

Rather than implementing the underlying OMAPI protocol directly, user
programs should use the dhcpctl API or OMAPI itself. Dhcpctl is a
wrapper that handles some of the housekeeping chores that OMAPI does
not do automatically. Dhcpctl and OMAPI are documented in dhcpctl(3)
and omapi(3). Most things you'd want to do with the client can
be done directly using the omshell(1) command, rather than
having to write a special program.

THE CONTROL OBJECT

The control object allows you to shut the client down, releasing all
leases that it holds and deleting any DNS records it may have added.
It also allows you to pause the client - this unconfigures any
interfaces the client is using. You can then restart it, which
causes it to reconfigure those interfaces. You would normally pause
the client prior to going into hibernation or sleep on a laptop
computer. You would then resume it after the power comes back.
This allows PC cards to be shut down while the computer is hibernating
or sleeping, and then reinitialized to their previous state once the
computer comes out of hibernation or sleep.

The control object has one attribute - the state attribute. To shut
the client down, set its state attribute to 2. It will automatically
do a DHCPRELEASE. To pause it, set its state attribute to 3. To
resume it, set its state attribute to 4.

SEE ALSO

AUTHOR

dhclient(8)
has been written for Internet Systems Consortium
by Ted Lemon in cooperation with Vixie
Enterprises. To learn more about Internet Systems Consortium,
see
http://www.isc.org
To learn more about Vixie
Enterprises, see
http://www.vix.com.

This client was substantially modified and enhanced by Elliot Poger
for use on Linux while he was working on the MosquitoNet project at
Stanford.

The current version owes much to Elliot's Linux enhancements, but
was substantially reorganized and partially rewritten by Ted Lemon
so as to use the same networking framework that the Internet Systems
Consortium DHCP server uses. Much system-specific configuration code
was moved into a shell script so that as support for more operating
systems is added, it will not be necessary to port and maintain
system-specific configuration code to these operating systems - instead,
the shell script can invoke the native tools to accomplish the same
purpose.